Bat and method of manufacturing the same

ABSTRACT

A bat comprises a bat body and a foamed core member provided inside the inner space of the bat body. The foamed core member is made of pre-foamed methacryl resin beads, which have been treated with a foaming process. The pre-foamed methacryl resin is filled up in the bat body so that approximately 100% of the volume is filled up. The foamed core member is formed by heating the pre-foamed methacryl resin beads in order to perform a secondary foaming process while the inner space is sealed. The bat has an expanded sweet-spot area and enhanced impact absorbing characteristics. By leveling the thickness of the bat body, mechanical strength is enhanced, and further, the weight of the bat can be reduced and satisfactory swing balance can be obtained.

CROSS-REFERENCES TO RELATED APPLICATION

This application claims priority from Japanese Patent Application Serial No. 2005-233105 filed Aug. 11, 2005, the content of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

Described herein is a bat used for playing a baseball game or a softball game, and a method for manufacturing the same.

BACKGROUND

Generally, a conventional bat for playing an amateur baseball game or a softball game is made from wood, FRP, or metallic material. Such a bat is manufactured by filling resin core material in a hollow bat body in order to enhance mechanical rigidity and impact absorbing characteristics of the bat and to control swinging balance thereof.

For example, Japanese Patent Laid Open Patent No H08-280860, discloses an FRP bat body in which foamed polyvinylidene chloride resin particles, foamed polypropylene resin particles, or foamed polystyrene resin particles are filled so that bat swinging balance can properly be controlled. Japanese Laid Open Patent No. 2001-79132, discloses that in order to secure as much repulsive characteristics as that of the conventional wooden bat, the inner space of an aluminum bat body is filled with a foamed DCPD (dicyclopentadiene) resin etc.

In the FRP bat described in Japanese Laid Open Patent No. H08-280860, the bat swing balance is controlled by adjusting specific gravity and injecting foamed resin material.

However, the foamable resin particles used as the core material of the conventional FRP bat have insufficient foamability and low rigidity. Hence, in the case of the FRP bat, if the foaming power is reduced so as to enhance the rigidity, it will in turn result in the increased weight of the resin core material, so that it is difficult to manufacture a practically usable bat. Further, the FRP bat filled with foamed resin material has a technical problem in that the ball flying distance and effective batting area can be hardly extended, i.e., the sweet-spot area can be hardly enlarged or the impact absorbing characteristics can be hardly enhanced.

In addition, in Japanese Patent No. 2001-79132, a conventional aluminum bat is disclosed, in which the aluminum bat has high repulsive characteristics and impact absorbing characteristics substantially equivalent to those of the wooden bats. In such a conventional aluminum bat, generally, the bat body is manufactured in a swaging molding process in the cold atmosphere. However, this process also involves another technical problem that the thickness of a portion from the tapered portion to the handgrip portion of the bat is increased, thereby increasing the weight of the bat, so that proper balance of the bat itself is deteriorated. Hence, conventional aluminum bat manufacturers have adapted measures to thin off the thickness of the bat by carrying out a swaging molding process after thinning off the thickness of the bat in a preliminary grinding process which is carried out to, for example, an area ranging from the tapered portion to the handgrip portion.

Nevertheless, in the process for manufacturing an aluminum bat, an aluminum tube as a base material is damaged or scratched in the grinding process so that the mechanical strength of the bat is lowered. Further, in order to precisely meet the criterion value, some conventional aluminum bat manufacturers need to perform a grinding process under precise control over the grinding amount, and since the tapered area to be processed is ground stepwise, so that a step-like portion is formed thereon.

SUMMARY

Accordingly, the bat according to embodiments set forth below solves at least one of the above technical problems of the conventional bats made from FRP material or aluminum etc. In the bat according to the embodiments, a sweet spot is enlarged and impact absorbing characteristics are enhanced, and the leveled thickness of the bat body is also secured so as to enhance the mechanical strength, and the weight of the bat is decreased so as to secure satisfactory swing balance for a user. A method for producing such a bat is also described.

The embodiments set forth below includes a bat comprising a bat body and a foamed core member formed with foamed resin beads filled up in the inner space thereof. Further, the bat body may be made of metallic material such as aluminum, aluminum alloy, titanium, or titanium alloy, or made from fiber-reinforced plastic (FRP) material, wherein the bat body has a cylindrical body having the closed tip portion and an opening at the grip end so that the entire body having the bottom portion extends along a longitudinal axis. The inner space of the cylindrical bat body may be filled with pre-foamed resin beads fed from the grip-end opening. The inner space of the bat body may be filled with pre-foamed methacryl resin beads which is treated with a predetermined foaming process in advance. By closing the grip-end opening, the inner space of the bat body filled with the pre-foamed methacryl resin beads may be tightly closed, and then, the bat body may be thermally treated to cause the resin beads to be expanded, thereby completing formation of the foamed core member.

The foamed methacryl resin material that forms the foamed core member has greater foaming capability, higher rigidity, and lighter weight than that of conventionally available foamed resin materials such as foamed vinylidene resin material, foamed polypropylene resin material, or foamed polystyrene resin material. For example, the foamed methacryl resin material has the rigidity about 1.8 times as high as that of the vinylidene resin material, and also has the weight about 0.7 times as large as that of the vinylidene resin material, in the same volume. Therefore, since the pre-foamed methacryl resin beads are filled up in the body of the bat so that approximately 100% of the volume of the inner space of the bat body is filled up, after the un-foamed methacryl resin beads are foamed so as to increase the volume thereof, for example twenty times as large as that of un-foamed beads, the amount of the methacryl resin beads to be filled in the bat body can be reduced, so that it is possible to reduce the weight of the foamed methacryl resin beads to be filled up. By the thermal treatment applied at a relatively low temperature within a short time after charging (filling up) the pre-foamed methacryl resin material, it is possible to execute a secondary foaming process, thereby generating substantial inner pressure to the bat body.

Accordingly, by reducing the weight of the foamed core member made of the foamed methacryl resin beads and by applying internally generated predetermined pressure to the bat body, a sweet spot can be enlarged, and the impact absorbing characteristics are enhanced.

Furthermore, the embodiments include steps of filling up pre-foaming methacryl resin beads into the inner space of the bat body from the opening formed in part of a grip end portion so that approximately 100% of the volume of the inner space is filled up with the pre-foamed methacryl resin beads, a step of sealing the inner space of the bat body, and a step of heating the pre-foamed methacryl resin beads to carry out a secondary foaming process inside the inner space by treating the bat body.

Further, in the method for manufacturing the bat, the process for manufacturing the bat body may comprise a step of forming a cylindrical base material, a step of carrying out a spinning rolling process, and a step of carrying out a swaging process before eventually forming a bat body having a predetermined shape provided with an opening at the grip-end portion. In the method, by using selected material, a cylindrical body material having the bottom portion may be formed in the step for manufacturing the bat body. Further, the method may includes a step of inserting a core metal member in the inner hole of the cylindrical material and a step of pressing a plurality of rollers of a rolling mill respectively disposed at different positions on the circumference of the cylindrical material in order to implement a spinning rolling process, and a step of forming the bat body material having an area corresponding to the grip-end portion having a predetermined thickness around the local area corresponding to the handgrip portion of the bat body material. According to the method, by applying a cold swaging process to the bat body material, a bat body having a predetermined shape may be eventually formed, wherein the handgrip portion is linked with the sweet-spot portion via the tapered portion so as to form a continual surface.

As described above, the foamed methacryl resin beads used for the foamed core member has greater foaming capability and higher rigidity than those of the conventionally available foaming resins such as foamed vinylidene resin, foamed polypropylene resin, or foamed polystyrene resin, and further the foamed methacryl resin beads are lighter than that of the others in the weight. For example, in the same volume, the foamed methacryl resin beads have the rigidity about 1.8 times higher than that of the foamed vinylidene resin, and has the weight about 0.7 times lighter than that of the foamed vinylidene resin. Accordingly, in the method for manufacturing a bat, by filling up the inner space of the bat body with pre-foamed methacryl resin beads whose volume is increased by about twenty times as large as the un-foamed methacryl resin beads so that approximately 100 % of the inner space of the bat is filled up with the pre-foamed methacryl resin beads, whereby the bat weight can be reduced. Further, in the method for manufacturing a bat, a plug member may be inserted and fixed to an opening formed on part of a handgrip portion so that the inner space of the bat body is fully closed, and then the foamed methacryl resin beads filled up in the closed inner space of the bat body are thermally treated so that a secondary foaming of the heated resin beads is carried out therein. The secondary foaming process may be carried out by the thermal treatment for a short time at a relatively low temperature. Accordingly, substantial predetermined inner pressure is generated from the inside of the body of the bat by the foamed core material, so that it is possible to manufacture a bat having an enlarged sweet-spot area and the enhanced impact absorbing characteristics.

Further, according to the method for manufacturing a bat, in the spinning rolling process, in a state in which the core metal has been inserted in the inner hole of the cylindrical material, the external circumference of the cylindrical metallic material is simultaneously pressed by rotating pressure rollers from three directions at a speed of, for example, 300˜600 rpm, so that while bringing the cylindrical material in contact with the core metal member, the cylindrical metal material is rolled in the axial direction so as to carry out a spinning rolling process thereby forming a base material for a bat body. Further, since in the method for manufacturing a bat, the controlling unit properly controls the pressing operation of the pressure rollers on the cylindrical metal material, it is possible to form a bat body material having an ideal thickness along the entire bat body in the axial direction without causing the grip-corresponding area to become thicker. Still further, in the method for manufacturing a bat, by performing a cold swaging process to the bat body material, it is possible to form a bat body having a predetermined shape with a continual surface ranging from the handgrip portion to the sweet spot portion through a taper portion. The cold swaging process is a method of reducing or forming steel or other material while cold, by drawing to a point or reducing the diameter, as may be required. Thus, since the bat body can be formed with the predetermined thickness throughout the entire bat body in the axial directional, it is possible to securely realize lighter weight and proper balance, and further, it is possible to enhance the rigidity as well.

As described above, the amount of foamed methacryl resin beads to be filled up in the inner space of the bat body is reduced by performing a predetermined foaming process for foaming the beads in advance, and the foamed methacryl resin beads are filled up in the inner space of the bat body so as to substantially fill the inner space of the bat body (approximately 100 % of the volume of the inner space), and further a secondary foaming process is carried out to the —pre-foamed methacryl resin beads in the state in which the inner space of the bat body remains closed. Consequently, it is possible to reduce the actual weight of the foamed methacryl resin beads and allow the bat body to be affected by substantial predetermined pressure generated from the inside of the bat body throughout a wide area thereof by the foamed core member, thereby making it possible to secure a wide sweet-spot area and enhance the impact absorbing characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the bat, and the method according to the present invention will be apparent from the ensuing description, taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a cross-sectional view of a bat according to an embodiment;

FIG. 2 is a cross-sectional view of a cylindrical material for producing a bat body according to an embodiment;

FIG. 3 is an explanatory diagram to show a process for treating cylindrical material in a spinning rolling process using spinning rollers;

FIG. 4 is a cross-sectional view of bat-body material treated in a spinning rolling process;

FIG. 5 is a cross-sectional view of a bat body formed by treating bat-body material in a cold swaging process;

FIG. 6 is a cross-sectional view of the bat body after treating the bat body material in a rounding process and a grip-end forming process; and

FIG. 7 is an explanatory diagram to show a process for filling a bat body with fine beads made from pre-foamed methacryl resin material.

DETAILED DESCRIPTION

While the claims are not limited to the illustrated embodiments, an appreciation of various aspects of the bat, and the method of manufacturing the same is best gained through a discussion of various examples thereof.

Referring now to the accompanying drawings, a bat 1 used for playing a baseball game or a softball game will be described in detail below. The bat 1 comprises a bat body 2, which is made from, for example, FRP (fiber-reinforced plastic), or lightweight metal material such as aluminum, aluminum alloy, titanium, or titanium alloy, and a foamed core material 3 (hereinafter also referred to as “core member 3”), which is formed from foamed resin beads enclosed inside an inner space 2A of the bat body 2. The bat 1 is made from, for example, aluminum material. As will be described later, the bat body 2 is formed into a predetermined shape after being treated with some processes including a process for forming a cylindrical bat body material, a process for applying a spinning rolling treatment to the cylindrical bat body material, and a process for treating the cylindrical bat material with a swaging process.

As shown in FIG. 1, the bat body 2 comprises a sweet-spot portion 4 having a slightly thick and even axial diameter and a predetermined length, in which a tip portion of the bat is closed, a tapered portion 5 that is gradually tapered from the sweet-spot portion 4 towards the bottom-end portion, a handgrip portion 6 that has a predetermined small diameter and continually extends from the tapered portion 5, and a grip-end portion 7 that is outwardly projected from a circumference of the end portion of the handgrip portion 6. An opening 8 is formed at the grip-end portion 7 of the bat body 2. By inserting a plug member 9 made from nylon resin material in the opening 8, the tightly sealed inner space 2A is formed.

Further, in the bat body 2, the tip portion of the sweet-spot portion 4 is tightly closed, and the opening 8 is formed at the grip-end portion 7, thereby the entire bat body 2 is formed into an approximately cylindrical configuration having a longitudinal axial and a bottom end therewith. The bat body 2 is entirely formed with a substantially even thickness, and the inner space 2A is formed, in which pre-foamed resin material is filled up so as to form the foamed core member 3 throughout the entire length of the bat body 2. The foamed core member 3 has a large thickness and is formed in the inner space 2A having a large inner diameter at the sweet-spot portion 4, and further, the foamed core member 3 is also formed at the handgrip portion 6 via the tapered portion 5.

Foamed methacryl resin beads 15 are treated in a predetermined foaming process in advance so that the volume of the material is increased, for example, about 20 times as much as that of the original foamable beads are used as the foamed core material 3. The methacryl resin has greater foaming characteristics and rigidity, than those of foaming vinylidene resin, foaming polypropylene resin, or foaming polystyrene resin, respectively, wherein the methacryl resin further has lighter weight than those resins. The methacryl resin has the rigidity about 1.8 times as high as that of the foamable vinylidene resin and has the weight about 0.7 times as large as that of the foamable vinylidene resin, in the same volume, thus exhibiting substantial foaming characteristics. Practically, methacryl resin bead powder having the above characteristics is used as the foamable methacryl resin beads 15. Approximately 100 % of the volume of inner space 2A of the bat body 2 is filled up with the methacryl resin beads powder from the opening 8 of the grip-end portion 7.

As described in detail later, by applying a predetermined thermal treatment to the bat body 2 in a state in which the inner space 2A remains sealed after inserting the plug member 9 in the opening 8, the foamed methacryl resin beads 15 are treated with a secondary foaming process before eventually forming the foamed core member 3. By causing the foamed methacryl resin beads 15 to be expanded more than the approximately 100 % of the volume within the inner space 2A of the bat body 2, the internal pressure of the inner space 2A is further increased.

As described above, in the bat 1 having the above structure, since the entire bat body 2 including the tapered portion 5 and the handgrip portion 6 is formed so as to have an even thickness, it is possible to reduce the weight of the bat body 2 and optimize the weight balance. Further, in the bat 1, the foamed methacryl resin beads 15 whose volume is increased by about 20 times as large as that of the un-foamed beads by pre-foaming are used as the foamed core member 3, and even when a large space is formed in the inner space of the bat body 2, i.e., even when a large sweet spot portion 4 is formed, it is possible to prevent increasing of the weight and to form the foamed core member 3 having the high rigidity throughout the entire length thereof. In the bat 1, since the pre-foamed methacryl resin beads 15 having high foaming characteristics are thermally treated so as to perform a secondary foaming process to the bat body 2 in the state where the inner space 2A thereof is tightly closed, it is possible to properly form the foamed core member 3 capable of increasing the internal pressure of the bat body 2.

In the bat 1, a repulsive force of the bat 1 can be increased by generating the high internal pressure by the foamed core material 3. In the bat 1, since the foamed core member 3 is formed by fully filling up the entire inner space 2A of the bat body 2 with the foamed methacryl resin beads 15, it is possible for the bat 1 to give a player a batting sense and the ball flying distance similar to those obtained when a ball hits a sweet spot of a conventional bat, even though the ball actually hits the tip portion, the tapered portion 5 or the handgrip portion 6.

Thus, since the foamed core member 3 is formed inside a large area of the bat body 2 without increasing the weight of the bat 1, it is possible to expand the sweet-spot area 4. Further, in the bat 1, since the foamed core member 3 can increase the inner pressure of the bat body 2 so as to enhance the impact absorbing characteristics, it is possible to fully absorb substantial shock and vibration upon striking a ball by the bat 1. Hence, the bat 1 has the characteristics equivalent to those of a conventional wooden bat by the foamed core member 3, thereby lessening burden of a batter and decreasing potential damage incurring to the bat body 2, whereby the durability of the bat 1 is increased. Unlike any conventional metallic bat, in the bat 1 according to the embodiment, a sound absorber to be provided in the inner space of the bat body 2 can be eliminated.

The bat 1 is manufactured in a method comprising a process for manufacturing the bat body 2 and a process for forming the foamed core member 3 in the inner space 2A of the bat body 2. The process for manufacturing the bat body 2 comprises a step of forming cylindrical base material from material, for example, FRP, or metals such as aluminum, aluminum alloy, titanium, or titanium alloy, a step of treating the cylindrical material in a spinning rolling process, and a step of treating the rolled material in a swaging process before eventually completing the bat body 2. In the step for forming cylindrical base material of the process for manufacturing the bat body 2, a cylindrical material 10 shown in FIG. 2 is formed. The cylindrical base material 10 has a diameter substantially equivalent to the axial diameter of the sweet-spot portion 4, wherein the entire cylindrical base material 10 has a thickness substantially equal to that of the bat body 2.

Next, as shown in FIG. 3, in the process for manufacturing the bat body 2, the cylindrical base material 10 is treated in a rolling process by using a spinning rolling mill, thereby reducing the diameter of the base material 10 to a predetermined axial diameter so as to form a bat body material 11 shown in FIG. 4, without thickening local area corresponding to the tapered portion 5 or the handgrip portion 6. As shown in FIG. 3, the spinning rolling mill comprises a core metal member 12 having a predetermined shape, which is inserted into the inner hole of the cylindrical base material 10, and a plurality of rollers 13 a to 13 c, in which the rollers 13 a to 13 c are brought in contact at different positions of the circumferential portion of the cylindrical base material 10. By pressing individual rollers 13 a to 13 c onto the circumferential portion of the cylindrical base material 10, respectively while the cylindrical base material 10 is rotated, the spinning rolling mill rolls a local portion H corresponding to the tapered portion 5 and the handgrip portion 6 of the cylindrical base material 10 so as to form a predetermined shape.

In order to facilitate removal of the processed base material, the core metal member 12 has a tapered portion(s).

In the spinning rolling process, while rotating the cylindrical base material 10 at a speed of 300 to 600 rpm, the rollers 13 a to 13 c are pressed onto the cylindrical material 10 with a predetermined pressure and the cylindrical material 10 is moved in the axial direction thereby rolling the local portion H as described above. In the spinning rolling process, for example, a motion controller appropriately controls the pressing operation and the moving operation of respective rollers 13 a to 13 c, so that it is possible to efficiently and precisely process local portions corresponding to the tapered portion 5 and the handgrip portion 6 that are continually extended from the sweet-spot portion 4 of the cylindrical base material 10, so that the cylindrical base material 10 is formed into a predetermined shape. Since it is possible to form the cylindrical material 10 into a predetermined shape without incurring scratches or damages at all while the spinning rolling process is carried out, the mechanical strength of the bat body 2 can securely be improved.

By implementing the spinning rolling process, it is possible to form a bat body material 11 having a predetermined shape in the state where the predetermined thickness of the local portions corresponding to the tapered portion 5 and the handgrip portion 6 is preserved. However, as shown in FIG. 4, the sweet-spot portion 4, the tapered portion 5, and the handgrip portion 6 of the bat body material 11 have not been formed into the shape of the bat, yet.

Hence, in the process for manufacturing the bat body 2, the bat body material 11 is processed in a conventional cold swaging process, so that a finish-up rolling process is carried out whereby the sweet-spot portion 4 and the handgrip portion 6 form a continuous surface via the tapered portion 5 before eventually forming the bat body 2 having a predetermined shape shown in FIG. 5. Although detailed description is omitted, in the process for manufacturing the bat body 2, a rounding process for closing the tip portion and an ensuing process for forming the grip-end portion 6 at the end of the handgrip portion 7 are respectively carried out after the cold swaging process.

After the process for manufacturing the bat 1, in which the bat body material 11 is formed, a step of filling up pre-foamed methacryl resin beads 15 which are still foamable, into the inner space 2A of the bat body 2 prepared through the above processes, a step of inserting the plug member 9 into the opening 8 and fixing it, and a step of carrying out the secondary foaming process so as to foam the pre-foamed methacryl resin beads 15 thereby forming the foamed core member 3, are carried out, thereby completing the bat 1. In the step of filling up the pre-foamed methacryl resin beads 15 into the inner space 2A of the bat body 2, as shown in FIG. 6, pre-foamed methacryl resin beads 15 having excellent characteristics are filled into the inner space 2A from the opening 8 formed at the grip-end portion 7. In the process for manufacturing the bat 1, since the pre-foamed methacryl resin beads 15 which are still foamable are filled into the inner space 2A of the bat body 2 via the opening 8 formed on the side of the grip-end portion 7, unlike any conventional structure designed for filling up resin material from the tip side of the bat whose diameter is large at the tip side, it is possible to enhance the mechanical strength of the bat body 2 by the method according to the embodiment.

Before the process for filling up the pre-foamed methacryl resin beads powder 15, the foaming process is carried out by applying steam and heat to resin beads powder, so that the volume of the thermally treated resin beads powder 15 is increased by about 20 times as much as the original volume. After that, the inner space 2A of the bat body 2 is substantially fully filled with the expanded resin beads powder 15 so as to overflow from the opening 8 of the grip-end portion 7. In the process for filling the pre-foamed methacryl resin beads powder 15, if the inner space 2A is substantially fully filled with the not-yet-foamed resin beads powder (100 % of the volume of the inner space), the weight of the bat becomes heavy. However, by the previously executed foaming process, the actual weight of the resin material (bat) can be reduced.

In the process for manufacturing the bat 1, if the pre-foamed methacryl resin beads 15 having a lower foaming power and a lighter weight than that of other foaming resin materials are used, despite the lighter weight, since more resin beads 15 are required to fill up the inner space of the bat, the volume of the pre-foamed resin beads 15 to be filled into the bat body 2 is increased, thereby increasing the weight of the bat 2. Conversely, if the foamable methacryl resin beads 15 having a much greater foaming power are used so as not to increase the amount of the resin beads to be filled in, despite the much greater foaming characteristics than that of other foaming resin materials, it will be difficult to generate sufficient internal pressure in the secondary foaming process. Accordingly, in the process for manufacturing the bat 1, pre-foamed methacryl resin beads powder whose volume is increased by about 10 to 30 times as much as the original foamable methacryl resin beads, preferably methacryl resin beads powder having the foaming characteristic of about 20 times as much as the original foamable methacryl resin beads is used.

In the process for manufacturing the bat 1, after inserting the plug member 9 into the opening 8 formed in the grip-end portion 7, the inserted plug member 9 is fixed by applying an adhesive agent, thereby fully sealing the inner space 2A of the bat body 2. In the condition, the secondary foaming process is carried out so as to foam the pre-foamed methacryl resin beads 15, wherein the secondary foaming process is executed by thermally treating the bat body 2 at, for example, 120° C. for about 30 minutes, thereby causing the pre-foamed methacryl resin beads 15 previously expanded by about 20 times as much as the original volume thereof are expanded inside the inner space 2A of the bat body 2 by the secondary foaming. The foamed methacryl resin beads 15 are expanded inside the inner space 2A while fully eliminating spaces between the beads, so as to increase the cubic volume and completing the foamed core member 3 capable of generating substantial inner pressure toward the bat body 2. In the process for manufacturing the bat 1, after completing the above serial processes including the secondary foaming process applied to the pre-foamed methacryl resin beads 15, the bat 1 having the foamed core member 3 formed inside the inner space 2A of the bat body 2 is produced.

In the process for manufacturing the bat 1, finish-up treatment such as a coating process and an engraving process is carried out to the bat 1 before filling up the inner space 2A with the pre-foaming methacryl resin beads 15. After fully completing the above serial processes, the commercially available bat 1 having a slip-proof bandage wound around the circumference of the handgrip portion 6 is eventually completed.

Embodiments of the present invention have been described in detail with reference to the attached drawings. The preceding description has been presented only to illustrate and describe exemplary embodiments of the bat and the method of the claimed invention. It is not intended to be exhaustive or to limit the invention to any precise form disclosed. It will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the claims. The invention may be practiced otherwise than is specifically explained and illustrated without departing from its spirit or scope. The scope of the invention is limited solely by the following claims. 

1. A bat comprising; a bat body made of metallic material or fiber-reinforced plastic material, wherein the bat body has a tip portion being closed and a grip-end portion having an opening; and a core member provided in an inner space of the bat body; wherein the core member made from foamed methacryl resin in which approximately 100% of a volume of the inner space is filled with the core member.
 2. The bat according to claim 1, wherein the metallic material includes aluminum, aluminum alloy, titanium, or titanium alloy.
 3. The bat according to claim 1, wherein the core member is formed by filling up pre-foamed methacryl resin in the inner space and carrying out a secondary foaming process to the pre-foamed methacryl resin in a state where the opening is closed and the inner space is tightly sealed.
 4. The bat according to claim 3, wherein the pre-foamed methacryl resin is in form of beads or beads powder.
 5. A method of manufacturing a bat comprising the following steps of; filling up pre-foaming methacryl resin beads into an inner space of a bat body from an opening formed in part of a grip portion so that approximately 100% of a volume of the inner space is filled up with the pre-foamed methacryl resin beads; sealing the inner space of the bat body; and heating the pre-foamed methacryl resin beads to carry out a secondary foaming process inside the inner space by treating the bat body.
 6. The method of manufacturing a bat according to claim 5, wherein the step of sealing the inner space is carried out by inserting a plug member into the opening of the bat body; and
 7. The method of manufacturing a bat according to claim 6, further including forming a cylindrical base material from the metallic material or the fiber-reinforced plastic material.
 8. The method of manufacturing a bat according to claim 7, further including forming a bat body material by fixedly inserting a core metal member into an inner hole of the cylindrical base material.
 9. The method of manufacturing a bat according to claim 8, further including pressing a plurality of rollers located at different positions of a circumference of the cylindrical base material, and performing a spinning rolling process for shifting the rollers in an axial direction of the cylindrical base material while rotating the cylindrical base material so as to form a predetermined thickness for a local area corresponding to the grip-end portion so that a bat body material is formed.
 10. The method of manufacturing a bat according to claim 9, further including performing a cold swaging process to the bat body material so as to form the bat body having a predetermined shape having a continuous surface linking a grip portion and a sweet-spot portion via a tapered portion.
 11. The method of manufacturing a bat according to claim 5, further including foaming un-foamed methacryl resin beads so as to prepare pre-foamed methacryl resin beads.
 12. The method of manufacturing a bat according to claim 11, wherein the a volume of the un-foamed methacryl resin beads is increased by approximately 10 to 30 times as much as the un-foamed foamable methacryl resin beads in the step of foaming un-foamed methacryl resin beads.
 13. The method of manufacturing a bat according to claim 12, wherein the a volume of the un-foamed methacryl resin beads is increased by approximately 20 times as much as the un-foamed foamable methacryl resin beads in the step of foaming un-foamed methacryl resin beads.
 14. The bat manufactured by a method comprising the following steps of: filling up pre-foaming methacryl resin beads into an inner space of a bat body from an opening so that approximately 100% of a volume of the inner space is filled up with the pre-foamed methacryl resin beads; sealing the inner space of the bat body by inserting a plug member into the opening of the bat body; and heating the pre-foamed methacryl resin beads to carry out a secondary foaming process inside the inner space. 